1) Field of the Invention
The present invention relates to sealed cells having non-resealable safety valves.
2) Description of the Related Art
In recent years, there has been a rapid reduction in the size and weight of mobile information terminals such as mobile telephones, notebook personal computers, and PDAs (Personal Digital Assistants). Higher capacity and higher energy density are required of cells and batteries serving as the driving power sources of such terminals. Non-aqueous electrolyte secondary cells such as lithium ion secondary cells have high energy density and high capacity, and as such are widely used as the driving power sources for the terminals. Especially popular among these are square or rectangular lithium ion secondary cells for their easy mountability in the devices.
When the lithium ion secondary cells, such as lithium ion secondary cells, are exposed to atmospheres of high temperature or unusually charged and/or discharged, the electrodes and the electrolytic solution react to each other and thus the electrolytic solution are decomposed, resulting in gas generation. When the gas increases the pressure inside the cell (or internal pressure) to or over a given level, there is the risk of cell burst. In view of this, cells are provided with safety mechanisms to release the gas outside the cells as soon as the internal pressure reaches or exceeds a given level. Such safety mechanisms need to be simple in structure and should not add to the weight of the cells. The techniques proposed to meet such demands include one of providing in the sealing plate a non-resealable safety valve (valve structure) that upon increase of the internal pressure, is broken or burst to form a break opened orifice. Such a technique is disclosed in the following Patent Documents 1 to 4.                Patent Document 1: Japanese Unexamined Application Publication No. 63-167669        Patent Document 2: Japanese Patent Publication No. 3222418        Patent Document 3: Japanese Unexamined Application Publication No. 2001-325934        Patent Document 4: Japanese Unexamined Application Publication No. 10-261391        
i) Patent Document 1 discloses the formation of, in part of a terminal plate, a thin portion formed by sectionally-wedge-shaped and coaxial-circular grooves wherein the circles have different diameters and the grooves are alternately provided on the inner and outer surfaces of the thin portion, so that the tops of the neighboring wedges are opposite. Such a thin portion is broken so as to permit the gas inside the cell (or internal gas) and solved substances to be externally released prior to the burst of the cell itself. Enabling to smoothly release the internal gas and solved substances to ambient atmosphere in this manner, this document claims to add to the safety of cells. With this disclosure, however, the stress relative to the internal pressure cannot be sufficiently concentrated on the sectionally-wedge-shaped grooves, which often causes variances in the values of pressure at which the thin portion is broken. Furthermore, the provision of the sectionally-wedge-shaped grooves renders the cell less resistant to impacts; the circular grooves can be easily damaged by the impacts of, for example, dropping, resulting in solution leakage. Thus, this disclosure can be problematic.
ii) Patent Document 2 discloses a sealing plate for sealing an opening portion of the container of a sealed cell. The sealing plate is constructed of a metallic plate and has a groove that is formed in the surface of the sealing plate along a contour surrounding a predetermined area. A swelling part that swells outward is formed within and continuously with the groove. The groove of the sealing plate is ruptured when pressure within the container becomes excessive and the pressure concentrated on the swelling part exceeds a predetermined level. This opens the swelling part and realizes communication with the outside. Although this document claims to add to the safety of cells with this disclosure, the groove and valve are still vulnerable to impacts and hence cracks are easy to occur, resulting in solution leakage. Thus, this document can also be problematic.
iii) Patent Document 3 discloses a thin-valve structure formed in a sealing plate for sealing a cell. The thin-valve structure is ruptured when the internal pressure reaches or exceeds a predetermined value and thus permits the internal gas to be externally released. The thin-valve structure has a dome-shaped dome and a break groove for facilitating the rupture of the thin-valve structure. The break groove is formed at the periphery of the dome, and a portion of the break groove is located in the middle of the thin-valve structure or near the middle of the thin-valve structure. Such a cell safety valve ensures a sufficient open area during the operation of the safety valve while reducing variances in the operating pressure of the safety valve. Furthermore, the leakage of the electrolytic solution is prevented. However, further improvement is desired in respect of the impact resistance of the break groove.
iv) Patent Document 4 discloses a V-shaped groove for preventing a cell from being ruptured. The V-shaped groove is located on an inner surface of a battery lid. According to this document, when the internal pressure exceeds a predetermined level, the pressure is released immediately. With this disclosure, however, the stress relative to the internal pressure still cannot be sufficiently concentrated on the V-shaped groove, causing variances in the values of pressure at which the safety mechanism is activated. Furthermore, the groove can be broken by the impacts of, for example, dropping, causing solution leakage. Again, this document can be problematic.